Here’s two potential scenarios we’ve heard. Treat these as anecdotal rumors for now, as we don’t know how realistic they are.

Apple might allow users to select two apps that can run in the background.

Apple might selectively allow some apps to run in the background. We assume that developers could apply for permission to run in the background, and that Apple might approve or deny them based on the resources they need and how well they behave with the operating system’s stability.

Ordinarily I wouldn’t link to something as sketchily sourced as this, but: I heard something very similar from a decent (but second-hand) source back in January during Macworld Expo. What I heard then was that Apple was working on a vastly improved dock for your most-frequently used apps, and that there’d be one special icon position where you could put a third-party app to enable it to run in the background. Take it with a grain of salt, though: my source in January described it as an idea Apple was working on, nothing more.

The major limiting factor right now is RAM. There just isn’t much left for third-party processes on the current hardware’s 128 MB.